Pressure and Differential Pressure Transmitters

What Is a Pressure Transmitter?

A pressure transmitter is a field device that senses the pressure of a fluid and converts it into a standard electrical signal and digital data. Unlike a mechanical pressure gauge, which only gives a local reading, it passes the measured value to a remote control system; smart transmitters additionally offer digital communication, local configuration, self-monitoring and process diagnostics. Pressure data can therefore be used not just for monitoring but also for control loops, alarms and safety functions.

There are three basic types, defined by the measurement reference: a gauge pressure transmitter measures relative to atmospheric pressure, an absolute pressure transmitter relative to full vacuum, and a differential pressure transmitter measures the difference in pressure between two process points. Differential pressure measurement is the basis of flow measurement with primary flow elements, of hydrostatic level measurement and of pressure-drop monitoring across equipment such as filters or heat exchangers.

The modular platform covered on this page combines, within a single electronics architecture, a flange-type sensor module suited to integration with manifolds, primary flow elements and diaphragm seals, and an in-line sensor module that connects directly to the process. Level, interface and density applications with diaphragm seals are covered separately on the differential pressure level transmitters and diaphragm seal systems page.

How Gauge, Absolute and Differential Pressure Measurement Works

In a pressure transmitter, process pressure is measured through a four-stage chain that runs from the wetted isolating diaphragm to the sensor and on to the digital electronics:

1.      Pressure acting on the isolating diaphragm: The process fluid applies pressure to a thin isolating diaphragm made from materials such as 316L stainless steel, Alloy C-276, Alloy 400 or tantalum. The diaphragm is the wetted barrier that keeps the process fluid away from the sensing element.

2.      Transfer to the sensor through the fill fluid: The sealed volume between the diaphragm and the sensing element is filled with silicone as standard or, optionally, with an inert fill fluid. Because volumetric displacement in the flange-type module remains below 0.08 cm³, pressure reaches the sensor without any appreciable change in volume at the diaphragm.

3.      Conversion to an electrical signal and digital processing: The sensing element converts pressure into an electrical quantity. The electronics refresh the sensor data 48 times per second (167 times with the high-speed option) and apply calibration and characterization data to produce the measured value in engineering units.

4.      Output, display and communication: Depending on the user’s choice, the measured value is converted into a two-wire 4–20 mA signal with a linear or square-root characteristic, with the HART digital signal superimposed on the current. Damping can be set between 0 and 60 seconds; the same measurement also feeds the local graphical display and, where fitted, the integrated relays.

Three Pressure Types by Measurement Reference

In gauge pressure measurement, the reference side of the sensor is open to atmosphere; with in-line modules the measurement extends to negative gauge values down to –1.01 bar, so near-vacuum conditions can also be monitored. In absolute pressure measurement, the reference is full vacuum; the reading is therefore unaffected by changes in atmospheric pressure, which makes it the preferred choice for vacuum processes and for monitoring quantities such as vapor pressure.

In differential pressure measurement, the high- and low-pressure sides of the transmitter are connected to two separate process points. Depending on the range code, the transmitter stays within specification at static line pressures up to 250 bar; with the high static pressure option this limit rises to 425 bar. The effect of line pressure on the zero point can be eliminated by performing a zero trim with the transmitter under line pressure. If incorrect valve operation exposes one side to full static pressure, the output shifts and the transmitter has to be re-zeroed.

Flange-Type and In-Line Sensor Modules

The flange-type sensor module handles all three measurement types: differential, gauge and absolute pressure. Its process connection consists of ¼–18 NPT ports on 54 mm centers, which flange adapters convert to ½–14 NPT. The module can be combined with 2-, 3- or 5-valve integrated instrument manifolds, traditional flanges and level flanges for direct tank mounting (2 in / 3 in Class 150–300, DN 50 / DN 80 PN 40).

The in-line sensor module is intended for gauge and absolute pressure only and, thanks to its compact design, mounts directly on the process connection. Options include ½–14 NPT female, G½ A DIN 16288 male, a non-threaded instrument flange and, for the highest range up to 689.47 bar, a coned-and-threaded high-pressure connection.

Flow and Level Measurement with Differential Pressure

The differential pressure across a primary flow element is proportional to the square of the flow rate; the transmitter’s square-root output or internal flow configuration turns this relationship into a linear flow signal. The platform is also offered as a complete DP flow meter, configured and leak-tested at the factory: an averaging pitot tube with low permanent pressure loss for large lines, a conditioning orifice that cuts the straight-pipe requirement to 2D upstream and downstream of flow disturbances, a compact orifice fitted between existing flanges, and an integral orifice with a precision-machined pipe section for small line sizes.

Depending on the primary element and performance class, flow measurement uncertainty lies between roughly ±0.75% and ±2.70% of flow; these figures assume constant density, viscosity and expansion factor and exclude pressure and temperature compensation. In level applications, the same differential pressure principle is applied to hydrostatic level measurement, either with a flange mounted directly on the tank or with diaphragm seal systems.

Diagnostics and Process Information

Electrical loop diagnostics recognize problems such as corroded terminals, water in the terminal compartment or an unstable power supply before they affect the process measurement. Impulse line blockage detection applies statistical signal processing to identify plugging in the impulse piping or at the pressure taps of the primary element.

A statistical process monitoring function tracks how much the measured pressure scatters (its standard deviation and its coefficient of variation) and can give early warning of abnormal conditions such as flame instability in furnaces, cavitation in pumps, flooding of distillation columns, changes in fluid composition, air entrainment, loss of agitation and process leaks. Guided proof-test menus, on-site partial proof testing that needs no external pressure source, and calibration and diagnostic logs make periodic testing of safety functions easier.

The transmitter meets the NAMUR NE 107 self-monitoring and NE 43 signal level recommendations. When a critical device failure is detected, the analog output is driven, according to the selected alarm configuration, to a high alarm level of 22.5 mA or a low alarm level of 3.725 mA (3.575 mA in the NAMUR-compliant configuration).

Key Features

·         Three performance classes: Reference accuracy of ±0.025% of span in the high-performance class and ±0.035% of span in the standard class; the flow-optimized class achieves ±0.04% of reading up to an 8:1 DP turndown.

·         Wide rangedown: A rangedown of 200:1 or 150:1 depending on performance class, and 800:1 between –2.49 and 2.49 bar with the universal range option, reduces the variety of spare instruments held for different ranges.

·         20-year long-term stability: Stability specified over 20 years as ±0.10% of the upper range limit in the high-performance class and ±0.15% in the standard class.

·         Fast dynamic response: Total response time of 80 ms in the standard configuration and 40 ms with the high-speed sensor option; dead time of 40 ms or 20 ms respectively.

·         Integrated relay switches: Two double-pole, double-throw relays on the terminal block, driven by the transmitter measurement, provide discrete control where no PLC or DCS is available and a redundant path to the final control element in safety instrumented systems.

·         Tiered diagnostic packages: Three levels of diagnostic coverage, from basic device and electrical loop diagnostics to guided proof testing, statistical process monitoring and impulse line blockage detection.

·         Local access via Bluetooth: Configuration and maintenance from a typical distance of at least 15 m in line of sight, reducing the need for personnel to enter hazardous locations.

·         Graphical display and remote interface: 128 × 128 pixel backlit graphical LCD, seven language options and local buttons for zeroing, re-ranging and loop testing; if required, a remote display with cable lengths up to 152.4 m.

·         Application-oriented configuration: In a few steps, the transmitter can be configured to measure level, volume, flow or totalized flow.

·         High static pressure capability: Static line pressure up to 250 bar in differential pressure measurement, or up to 425 bar with the high static pressure option.

·         Integrated manifold and connection options: Assemblies with 2-, 3- and 5-valve integrated manifolds, traditional flanges and level flanges can be ordered as a single unit.

·         Cold-climate options: Options for operation at ambient temperatures of –50 °C and –60 °C in suitable configurations.

Technical Specifications

The values below are typical technical data for the pressure and differential pressure transmitter platform described on this page. They vary with measurement type, range code, performance class and selected options, and should be confirmed at project stage.

Line pressure and ambient temperature effects, mounting position effect (max. 3.11 mbar for flange-type differential and gauge modules, can be zeroed out) and overpressure limits vary with range code. In assemblies made up of several components, the maximum working pressure is limited by the component with the lowest pressure rating.

Key Advantages

·         Fewer spare instrument variants: A wide rangedown allows one transmitter to be re-ranged for different spans, reducing the number of models kept in stock.

·         Long-term measurement reliability: Stability specified over 20 years supports risk-based planning of calibration verification intervals and helps reduce drift-related errors.

·         Avoiding unplanned shutdowns: Electrical loop, impulse line blockage and statistical process monitoring diagnostics make problems that affect the measurement visible before they turn into failures.

·         Safer maintenance for field personnel: Bluetooth connectivity and a remote display help staff carry out configuration and checks without entering the hazardous area or climbing to hard-to-reach points.

·         Suitability for safety functions: IEC 61508 certification, FMEDA data and on-site partial proof testing support design and periodic testing processes in safety instrumented systems.

·         Fast control loops: A response time as low as 40 ms and sensor updates 167 times per second improve control quality in fast-changing pressure and flow loops.

·         Resistance to harsh site conditions: An IP66/IP68 housing, vibration resistance tested to IEC 61298-3, EN 61326 EMC compliance and an optional surge protection unit maintain measurement continuity at electrically and mechanically demanding sites.

·         No separate pressure switch required: Integrated relays provide discrete control based on a pressure, flow or level variable from the same device, reducing the accuracy and reliability issues associated with separate pressure switches.

Application Areas

Pressure and differential pressure transmitters are used for process control, flow and level measurement, equipment monitoring and safety functions. Flow measurement technologies other than DP can be evaluated separately under flow measurement solutions.

Oil, Gas and Refining

In sour oil and gas production and refinery environments, differential pressure is measured under high static pressure with wetted materials that meet NACE MR0175/ISO 15156 and MR0103/ISO 17945 requirements. DP flow meters integrated with a primary element can be used for liquid, gas and steam flow.

Chemicals and Petrochemicals

For aggressive media, Alloy C-276, Alloy 400 or tantalum diaphragms are preferred. Statistical process monitoring helps detect conditions such as distillation column flooding, changes in fluid composition or loss of agitation at an early stage.

Power Generation and Steam Systems

DP flow measurement is performed on steam and feedwater lines, and pressure is monitored in boiler and furnace systems. Tracking the variability of the pressure signal provides additional information on abnormal conditions such as furnace flame instability and pump cavitation.

Safety Instrumented Systems

Transmitters supplied with IEC 61508 certification and FMEDA data can serve as pressure, flow and level inputs in safety instrumented functions; the integrated relays can provide a redundant path to the final control element.

Water and Drinking Water Plants

With the drinking water approval option, pressure, filter differential pressure and flow can be measured on treatment and distribution lines. At pump stations without a PLC, the integrated relays can be considered for simple discrete control.

Marine and Offshore Installations

Type approval options from classification societies allow the transmitter to be used for pressure and level measurement on ships and offshore platforms; a stainless steel housing may be preferred in corrosive atmospheres.

How to Select a Pressure or Differential Pressure Transmitter

Choosing the right transmitter means assessing the process and control requirements of the measuring point together with the instrument specifications. For a technical assessment we recommend clarifying the following:

Measurement Type and Range

·         Variable to be measured: gauge, absolute or differential pressure; whether flow or level is to be calculated

·         Minimum, normal and maximum operating pressure and the required calibrated span

·         For differential pressure: maximum static line pressure and possible one-sided overpressure

·         Operation under vacuum or negative gauge pressure

·         Expected range changes and the rangedown required

Accuracy and Dynamic Performance

·         Required reference accuracy and performance class

·         Total performance under ambient temperature changes and line pressure

·         Speed requirement of the control loop (standard or high-speed response)

·         Long-term stability and planned calibration verification interval

·         For DP flow applications: primary element type and target flow uncertainty

Process Connection and Materials

·         Process temperature, fill fluid (silicone or inert) and vacuum service

·         Wetted-material compatibility and NACE requirements for sour service

·         Connection arrangement: integrated manifold, traditional flange, level flange or direct in-line connection

·         Impulse line length and plugging risk; a diaphragm seal system if necessary

·         Cleaning and pressure test options for services that require special cleaning

Electrical Interface, Communication and Certification

·         4–20 mA HART integration, relay output and preferred NAMUR alarm level

·         Need for a local display, remote display and Bluetooth

·         Housing material and conduit entry thread (½–14 NPT, G½, M20 × 1.5)

·         Hazardous-area classification and the set of Ex approvals required

·         Functional safety (IEC 61508), material traceability (EN 10204 3.1) and calibration certificate requirements

In corrosive, viscous, plugging-prone or very hot processes, the transmitter should be used with a diaphragm seal system instead of being connected directly to the process; selection is then carried out for the complete system, including the seal and fill fluid.

Hazardous-Area, Functional Safety and Material Certificates

Depending on the option codes selected, the following approvals, certificates and standards are available for the transmitter:

The scope of each approval depends on the option code ordered; the hazardous-area and functional safety certificate set applicable to a specific project should be confirmed together with the TLY Enerji engineering team.

Pressure Measurement Solutions from TLY Enerji

TLY Enerji approaches pressure and differential pressure measurement not as the supply of a single instrument but as the design of the complete measuring point. Starting from operating and design pressures, our engineering team defines the measurement type, range and performance class, reviews the impulse line and manifold arrangement and, in DP flow applications, supports the joint sizing of the primary element and the transmitter.

Throughout the project we provide technical support for documenting hazardous-area and functional safety requirements, transferring HART data and diagnostic alarms to PLC, DCS or SCADA systems, integrating relay outputs into the control logic and setting zero and span during commissioning. We also help plan calibration verification intervals based on the instrument’s stability data and the criticality of the process.

Frequently Asked Questions

What is a pressure transmitter and how does it differ from a pressure gauge?

A pressure transmitter is a field device that measures fluid pressure and converts it into a standard electrical signal such as 4–20 mA and into digital data such as HART. A pressure gauge only displays the measurement locally, whereas a transmitter sends it to the control system; smart transmitters also offer remote configuration, alarms, relay outputs and device and process diagnostics. This is why transmitters are used in control loops and safety functions.

What is the difference between gauge, absolute and differential pressure transmitters?

A gauge pressure transmitter measures relative to atmospheric pressure, so its reading changes as atmospheric pressure changes. An absolute pressure transmitter uses full vacuum as its reference and is suitable for vacuum processes and measurements that must not be influenced by atmospheric variations. A differential pressure transmitter measures the pressure difference between two process points and is used for flow, level and filter or heat-exchanger pressure-drop applications.

How is flow measured with a differential pressure transmitter?

A primary element installed in the line — such as an orifice plate, a conditioning orifice or an averaging pitot tube — creates a differential pressure related to the flow. Because this differential pressure is proportional to the square of the flow rate, the transmitter performs a square-root calculation to produce a linear flow signal. Flow uncertainty depends on the primary element and turndown; for gas and steam, pressure and temperature compensation should also be considered.

What does rangedown mean for a pressure transmitter?

Rangedown is the ratio between the transmitter’s upper range limit and the smallest calibrated span that can be set. For example, a transmitter with a 200:1 rangedown can be set to a span as narrow as 0.5% of its upper range limit. A wide rangedown allows the same instrument to be used for different ranges and reduces the variety of spares in stock; however, accuracy formulas may change at narrow spans, so a calculation should be made.

How does long-term stability affect calibration planning?

Long-term stability describes how far a transmitter may drift from its reference value over time. For the platform described on this page, stability is specified over 20 years as ±0.10% or ±0.15% of the upper range limit, depending on performance class. Low drift makes it possible to plan calibration verification intervals on a risk basis, according to process criticality and the permissible error budget.

How can a blocked impulse line be detected?

When impulse lines become blocked, the transmitter can no longer sense the real pressure fluctuations in the process, and its output behaves as if frozen. Impulse line blockage diagnostics monitor the noise characteristics of the pressure signal statistically, treat significant changes in noise level as a sign of plugging and generate an alert. The same method also helps reveal blocked pressure taps on the primary element and other process connection problems.

How does static pressure affect differential pressure measurement?

In differential pressure transmitters operating under high static pressure, line pressure can cause small shifts in zero and span. The zero error can be removed by trimming the zero while the transmitter is under line pressure. The maximum static line pressure must always be specified during selection: on this platform the limit is up to 250 bar depending on the range code, rising to 425 bar with the high static pressure option. Re-zeroing may be necessary after incorrect valve operation.

What are integrated relay switches used for?

Integrated relays move the function of a conventional pressure switch into the transmitter. Operating on the transmitter’s own measurement and process alert settings, the two relays can directly switch pump, valve or alarm circuits based on a pressure, flow or level variable. This provides discrete control in installations without a PLC or DCS; the relays must be powered from a supply that is independent of the transmitter supply.

Can pressure transmitters be used in hazardous areas and safety functions?

Yes. The transmitter can be ordered with flameproof, intrinsically safe and dust protection types under ATEX and IECEx, as well as with US and Canadian approvals. For safety instrumented systems, an IEC 61508 certificate and FMEDA data are available as an option; guided proof-test and partial test functions make periodic testing easier. The appropriate approval set should be defined per project according to the hazardous-area classification of the site.